Number of Atoms in a Unit Cell

A corner atom is shared by 8 unit cells, so it contributes $\dfrac{1}{8}$ to one cell. A face-centre atom is shared by 2 unit cells, so it contributes $\dfrac{1}{2}$.
X atoms at 8 corners: $8 \times \dfrac{1}{8} = 1$ X atom per unit cell
Y atoms at 6 face centres: $6 \times \dfrac{1}{2} = 3$ Y atoms per unit cell
Ratio X : Y = 1 : 3
So the formula of the compound is $XY_3$.

A atoms at 8 corners: $8 \times \dfrac{1}{8} = 1$
B atoms at 6 face centres: $6 \times \dfrac{1}{2} = 3$
Ratio A : B = 1 : 3
So the formula is $AB_3$.